Influence of environmental parameters on the accuracy of nitrogen dioxide passive diffusion tubes for ambient measurement

Two studies at three sites ill the UK provided confirmation that systematic positive bias in NO2 diffusion tube measurement occurred because of change s to "within-tube" chemistry, rather than eddy diffusion at the mouth of th e tube. In the first study in Cambridge, UK, sampler overestimation for 1 a nd 2 week exposures was compared to corresponding time-averaged monitor mea surements (NO-NO2-NOx, O-3) and weather variables. Nonlinearity between sam pler and monitor NO2 measurements was interpreted in terms of spatial and t emporal variations in relative and absolute availability of NO, NO2 and O-3 , at the site. A maximum overestimation occurred for an exposure mean NO2/N Ox similar to 0.5. The separate contributions of reduced NO2 photolysis and eddy diffusion were compared in Study II using samplers of two materials, acrylic and quartz, and of different lengths (40, 55, 71 and 120 mm) at thr ee sites: Norwich background, Cambridge intermediate, London kerbside. For compared sites, NO2 measured by acrylic samplers was significantly higher t han for equivalent quartz samplers. For quartz samplers [NO2](mean) was onl y just above the monitor at Norwich and London; sampler/monitor NO2 = 1.04 (P = 0.59) and 1.01(P = 0.76), respectively. For acrylic samplers the order of [NO2](mean) was 40 mm > 120 mm > 71 mm greater than or equal to 55 mm. Excepting 40 mm samplers, this accords with a chemical bias where co-diffus ing NO and O-3 molecules in longer tubes have more time to react to form ex cess NO2. Bias in 40 mm samplers is discussed. Eddy diffusion is negligible for standard samplers because [NO2](mean) was equivalent for 55 mm and 71 mm acrylic samplers and close to monitor NO2 for 71 mm quartz tubes. Both s tudies showed that sampler accuracy was dependent on location. Significantl y, overestimation was greatest (similar to 3-4 ppb) where the NO2 annual me an was similar to 20 ppb, close to the UK and EU air quality standard of 21 ppb.